The Federal Communications Commission and cable television testing organizations such as Cable Labs, have been evaluating digital television delivery systems in order to choose a new television "standard" which someday will replace NTSC in the United States. These systems all involve digital coding and data compression techniques, for example those utilizing the MPEG algorithm or variations thereof. MPEG is discussed in U.S. Pat. No. 5,243,428 which is incorporated by reference herein.
Several systems, such as those demonstrated by the Advanced Television Research Consortium and the American Television Alliance, propose using quadrature amplitude modulation (QAM) or Vestigial Sideband (VSB) modulation techniques in the transmission of the coded television information.
The FCC plans to test and approve an HDTV standard for terrestrial broadcasting in 1995. Although the specifics of the standard are yet to be fully tested and agreed upon, the FCC has indicated that the system will initially take the form of a so called "simulcast" approach. The new HDTV signals will have to fit into currently unused television channels (so-called "taboo" channels) and initially co-exist with conventional analog television signals without co-channel interference.
NTSC will be used hereinafter to represent one example of conventional television broadcasting. Other examples would be SECAM and PAL. Although NTSC is exemplified herein, it is not meant to be construed as a limitation and will be used herein synonymously with "conventional" to represent conventional television in general.
The system proposed by the ATRC utilizes quadrature amplitude modulation (QAM)and a 32 point constellation, defining points each having a "real" value which is transmitted on a subcarrier "in phase" and an "imaginary" value which is transmitted 90 degrees out of phase (in quadrature) from the "real" value, to encode multibit code words of data into complex symbols transmitted on the channel.
Another digital transmission approach suggested by Zenith Electronics Corp. transmits just the "real" values of a constellation as a four level data signal which is transmitted using vestigial sideband modulation. This approach, known also as "4 VSB" is described in U.S. Pat. Nos. 5,086,340 and 5,087,975 which are incorporated by reference herein.
A discussion of both QAM and VSB digital modulation methods can be found in the article "Performance of Digital Modulation Methods", Communications Technology, May 1992. This article is incorporated by reference herein.
In order to minimize or prevent co-channel interference, the system chosen by the FCC will have to maximize the use of the available bandwidth. For this reason, systems proposed thus far have only proposed the transmission of 1050 interlaced lines of resolution or 787.5 progressive lines of resolution at a 60 frame/second rate.
Because of the co-channel interference due to the frequency re-use implied by "simulcasting", true high resolution progressive transmission will be difficult at a 60 frame/second rate. The FCC intends however, to end NTSC broadcasting at some, yet to be chosen, date after introduction of HDTV. It is expected that by the time a genuine market need for progressively scanned high definition video material is established, the NTSC transmissions will have been ended. At that time, all of the existing NTSC channels will be reassigned for HDTV service. This will remove the limitation currently in place on the transmission characteristics of the HDTV signal, allowing the migration from the proposed QAM systems (for example 32 QAM using a 32 point constellation QAM modem) to a higher capacity QAM system (for example a 128 QAM using a 128 point constellation QAM modem) which can be utilized to provide information which can augment the interlaced HDTV signal or provide high resolution progressively scanned programming. Similar migration is possible from a "simulcast" multilevel VSB system to a higher capacity VSB system (for example from a 4 VSB system to an 8 VSB system).
One object of the instant invention, is to design a transmission system which can be used as part of the FCC's "simulcast" standard while conventional broadcasting continues, but which can easily be upgraded to a higher level when "simulcast" ends.
Another object of the instant invention is to provide a television receiver which will receive the "simulcast" HDTV broadcast and remain compatible with future HDTV systems which will provide augmentation information to new generations of receivers capable of receiving and processing this information.
Another object is to design a transmission system which will enable HDTV set manufacturers to design television receivers today which will receive and display the "simulcast" HDTV signal, and also be easily upgradable so as to receive and display the higher level transmissions of the future.